FR-4, an abbreviation for Flame Resistant 4, is a type of material used for making a printed circuit board (PCB). It describes the board itself with no copper covering. The FR-4 used in PCB’s is typically UV stabilized with a tetrafunctional resin system. It is typically a yellowish color. FR-4 manufactured strictly as an insulator (without copper cladding) is typically a difunctional resin system and a greenish color. FR-4 is similar to an older material called G-10. G-10 lacked FR-4′s self extinguishing flammability characteristics. FR-4 has widely replaced G-10 in most applications. Some military appications where destruction of the circuitboard is a desirable trait will still utilize G-10.

Here’s more information on resins:
Resin is a hydrocarbon secretion of many plants, particularly coniferous trees, valued for its chemical constituents and uses such as varnishes and adhesives. The term is also used for synthetic substances of similar properties

The charring you are referring to is called "Remelt"
It is seen in the EDM process as well.
I am unfamiliar with the material you are referring to.
Can you offer more information?

Hair Loss Discoveries: Techniques of the Future